Electronic identification systems typically include two devices which are configured to communicate with one another. Preferred configurations of the electronic identification systems provide such communications via a wireless medium.
One such configuration is described in U.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996, assigned to the assignee of the present application, and incorporated herein by reference. This application discloses the use of a radio frequency (RF) communication system 10 including one or more radio frequency transponder modules 12, as described below with reference to FIG. 1. The transponder modules 12 are each coupled to an object (not illustrated) that the system 10 recognizes because of signals that the transponder modules 12 emit in response to stimuli. The system 10 includes an interrogator 13 operating in response to commands from a controller 14. Data and commands are exchanged between the interrogator 13 and the controller 14 through interconnections 15.
In one mode of operation, a transmitter TX 16 contained in the interrogator 13 supplies RF signals through interconnections 17 to a transmit/receive (T/R) antenna system 18. The T/R antenna system 18, in turn, radiates an interrogation signal 20 to one or more of the transponder modules 12. When the interrogation signal 20 is received by one of the transponder modules 12, a response signal 24 may be generated and transmitted. The response signal 24 typically includes modulation allowing some property or set of properties of the transponder module 12 to be determined. The property or properties describe the object (not illustrated) to which the transponder module 12 is coupled.
The response signal 24 is received by the T/R antenna system 18 and is coupled to a receiver RX 28. The receiver RX 28 demodulates the received response signal 24 and supplies information determined from the received response signal 24 to the controller 14 via the interconnections 15. The controller 14, in turn, may be able to supply information derived from the response signal 24 to an external processor (not illustrated) via a bus or other data link 30.
Systems such as the system 10 find wide-ranging application in a broad variety of settings. The IPASS toll collection system presently in use in the Chicago area is one example of such a system. In transponder-based toll collection systems, the presence of an object, such as a moving vehicle (not illustrated), is detected by a detector. In a toll collection system, the detection process may rely on reflection of electromagnetic waves, detection of magnetic anomalies or detection of a large mass.
In response to detection of the presence of the vehicle, the controller 14 causes the transmitter TX 16 to transmit interrogation signals 20 having carefully limited range. The transponder module 12 is typically a dash-mounted unit that has been pre-programmed to identify the vehicle and to provide information regarding a pre-existing account associated with that vehicle.
The transponder module 12 transmits the response signal 24 containing information to the interrogator 13. The information in the response signal 24 identifies the vehicle and the account associated with that transponder module 12. The receiver RX 28 receives the response signal 24, demodulates this information and couples the received information to the controller 14. The controller 14 may pass the received information to one or more computers (not shown) via the data link 30, causing the account associated with the vehicle to be appropriately decremented. As a result, the vehicle need not slow for the toll collection process, providing smoother traffic flow, reducing fuel consumption and generally facilitating both vehicular transportation and toll collection.
Similarly, an automated fuel retail system (not illustrated) may use the system 10 including the interrogator 13. The interrogator 13 exchanges signals with the transponder 12 that is attached to vehicle (not illustrated) to determine whom to bill for fuel dispensed to that vehicle when the vehicle is fueled at the fuel dispensing station.
In these kinds of systems 10, a range of the interrogation signal 20 is carefully limited to avoid interrogation of more than one transponder module 12 at a time. Additionally, it is often desirable to limit the amount of power that is required by the transponder module 12 in order to provide a stand-alone transponder module 12 that is able to operate without requiring an external source of power and that has long battery life.
Further, it is generally desirable to provide systems 10 that are as simple as possible. One approach to realizing these goals is to receive and transmit the interrogation 20 and response 24 signals in a common frequency band. In some systems, the response signals 24 are derived from the interrogation signals 20 and the transponder module 12 may even be one that simply modulates and re-transmits the received interrogation signal 20 to provide the response signal 24.
One simple technique for modulating the interrogation signal 20 to form the response signal 24 is to either (i) forward bias a diode that is coupled between ground and an antenna in the transponder module 12 to couple the antenna to ground, corresponding to a first logical state, or (ii) to leave the diode unbiased or reverse biased, decoupling the antenna from ground and corresponding to a second logical state. The antenna re-transmits the interrogation signal 20 in the second logical state but not in the first logical state. The transponder module 12 repeatedly switches between these two logical states according to predetermined patterns while the interrogation signal 20 is present to modulate the response signal 24 with a binary signal. These kinds of systems 10 are known as backscatter systems.
In some backscatter systems, the interrogation signal 20 includes a modulated preamble that carries data identifying which transponder module 12 in a population of such transponder modules 12 is targeted to respond. The preamble of the interrogation signal 20 further may include a request for specific data from the targeted transponder module 12. In such backscatter systems, the interrogation 20 and reply 24 signals must necessarily use the same frequency band and both the interrogation 20 and response 24 signals are present at the same time.
Backscatter RFID systems 10 are presently of great interest for identifying, sorting, counting and routing in situations where selected objects in a population of objects require individual recognition and treatment. Examples include luggage-handling and routing systems associated with public or private transportation systems, package handling and routing systems, vehicle or other rental or check-out systems and inventory control systems.
Some kinds of systems 10 may interrogate a large number of transponder modules 12 simultaneously. For example, an inventory control system may be used to determine if a specific item coupled to the target transponder module 12 is contained in a warehouse. Typically, each transponder module 12 is associated with an inventory item in the warehouse and vice versa. Various protocols have been developed to facilitate identification of, and communication with, a specific transponder module 12 in a population of similar transponder modules 12.
In many of these systems 10, there is need for low cost, compact and rugged transponder modules 12 that are secured to objects, such as vehicles, having metallic, and therefore conductive, surfaces. It is also desirable that the transponder modules 12 provide a compact package that does not protrude from the object bearing the transponder module 12. However, most antennas operate by developing electrical fields between one portion of the antenna and another portion of the antenna. When an antenna is mounted flush against a conductive surface, the potential differences that are required for successful operation of the antenna are effectively neutralized by image charges developed in the conductive surfaces of the object being tagged by the transponder module 12.
What is needed is an antenna structure that can be incorporated into a compact transponder module without compromising the form factor of the transponder module and that can operate effectively even when the transponder module is mounted on a conductive surface of the object bearing the transponder module.